Scotland should hold a new referendum on independence from the United Kingdom by 2021 if Britain leaves the European Union, the Scottish leader Nicola Sturgeon said Wednesday, even as she acknowledged that she lacked the power to make that happen on her own.

Ms. Sturgeon, Scotland’s first minister and the leader of the pro-independence Scottish National Party, has argued that Brexit changes the situation because Scotland should not be dragged out of the 28-member European Union against its will.

She told the Scottish Parliament in Edinburgh that if Britain leaves the European Union, “a choice between Brexit and a future for Scotland as an independent European nation should be offered in the lifetime of this Parliament” — before the next scheduled Scottish election, in May 2021.

Ms. Sturgeon said that the Scottish government would introduce legislation setting the framework for a new referendum. Holding such a vote, however, would need approval from the British government, which says the time is not right.

The British government’s secretary of state for Scotland, David Mundell, said Ms. Sturgeon “continues to press for divisive constitutional change when it is clear that most people in Scotland do not want another independence referendum.”

Ms. Sturgeon acknowledged the opposition from the Conservative government in London, but said, “I believe that position will prove to be unsustainable.”

“If we are successful in further growing the support and the demand for independence,” she added, “then no U.K. government will be able to stop the will of the people.”

Brexit, which had been scheduled to take place last month, has been delayed as Prime Minister Theresa May’s government has struggled to win Parliament’s backing for its European Union divorce agreement.